The increasing demand for wireless communication services such as provided by cellular and Wireless Local Loop (WLL) systems requires the operators of such systems to attempt to make a maximum effective use of the available radio frequencies. Consider, for example that a WLL system operator is typically allocated a geographic territory and a certain amount of radio bandwidth that affords the ability to transmit and receive on only a particular number of radio channels. In an effort to make the best use of the allocated frequency space the geographic territory is divided into a number of sub-areas called cells. A number of base stations are deployed throughout the assigned territory, with there typically being one base station in each cell. Transmission power levels are kept low enough so that the subscriber units, called Fixed Access Units (FAUs), in adjacent cells do not interfere with each other.
The system operator can then determine how to split up the allocated radio frequencies among the cells so that FAUs using the same frequencies do not interfere with one another. This process is intended to maximize channel availability in the service area, that is, to maximize the number of channels which may be used in a particular area at any one time. The object of this frequency planning process is to reuse each frequency as often as possible. Cells which reuse the same frequency set in this manner are referred to as homologous cells. In general, reusing a frequency in every Nth cell thus means that 1/N of all frequencies are available in any given cell. It is therefore usually desirable to select the reuse factor, N, to be as small as possible in order to increase the capacity for handling remote units in each cell.
However, conflicting with this requirement is a real world consideration that as the reuse factor N is decreased, the interference between channels in homologous cells increases. In other words, there is a design dilemma in that as N decreases, so does the distance between homologous cells, and thus the amount of interference between FAUs located in different cells but operating on the same frequency increases. The ratio between the RF power of a desired carrier signal, C, and the interference, I, created by FAUs operating in homologous cells is referred to as the carrier-to-interference ratio, C/I. Thus, as the reuse factor N decreases, it is generally understood that the C/I ratio is normally expected to increase.
What is needed is a way to decrease the channel reuse factor by partitioning the use of frequencies among cells without also necessarily imposing an increase in the carrier to interference ratio.